1. Field of the Invention
The present invention relates to a process for producing polyethylene by polymerizing ethylene or copolymerizing ethylene with other .alpha.-olefin in the presence of a highly active catalyst system, the preparation of which is simplified with use of a novel magnesium component.
2. Description of the Prior Art
It has been disclosed that when polymerization of ethylene or copolymerization of ethylene with other .alpha.-olefin (the polymerization and copolymerization will hereinafter generally be referred to as "polymerization of ethylene") is carried out in the presence of a catalyst system comprising a solid catalyst component obtained by reacting a reaction product of a hydropolysiloxane with a Grignard reagent, with a titanium or vanadium halide or an organoaluminum or a dialkylzinc, the yield of polyethylene per unit weight of the transition metal is extremely high, and the process as such is extremely advantageous for the industrial production of polyethylene (Japanese Examined Patent Publication No. 13232/80).
In the above Patent Publication, it is recommended that in order to remove an ether compound accompanying the Grignard reagent in the preparation of the reaction product of the hydropolysiloxane with the Grignard reagent, the reaction mixture is dried by evaporation and then dissolved in an aromatic hydrocarbon or an aromatic hydrocarbon having a boiling point higher than that of the ether compound is added to the reaction mixture and then the ether compound is removed by distillation to obtain the reaction product in a state as dissolved in the aromatic hydrocarbon. The amount of the ether compound remaining in the reaction product is influencial over the catalytic activity and it is therefore desired to minimize the remaining ether compound. However, according to the above mentioned methods, it is difficult to reduce the amount of the remaining ether compound to less than 0.4 mol per mol of magnesium in the reaction product. Namely, the reaction product of the hydropolysiloxane with the Grignard reagent is insoluble in an aliphatic hydrocarbon or an alicyclic hydrocarbon, and accordingly when such a hydrocarbon having a boiling point higher than the ether compound is added to the reaction mixture and the mixture is subjected to distillation in an attempt to remove the ether compound, the viscosity of the reaction mixture tends to gradually increase and the mixture becomes difficult to stir, or the reaction product tends to separate from the hydrocarbon and adheres to the wall of the distillation vessel, whereby it is impossible to properly carry out the distillation. Thus, it is thereby difficult to obtain the reaction product with a minimum amount of the remaining ether compound, in a form of a suspension.
It is generally known that when ethylene is polymerized in the presence of a Ziegler catalyst, an aliphatic hydrocarbon or an alicyclic hydrocarbon is usually used as the polymerization medium and the catalytic activity will be deteriorated if an aromatic hydrocarbon is incorporated. Therefore, when the aromatic hydrocarbon solution of the above reaction product is to be used as a catalyst component, it is essential to remove the aromatic hydrocarbon prior to supplying it to the polymerization reactor, and it is necessary to sufficiently wash the solid component obtained by the reaction with an excess amount of a transition metal compound, with an aliphatic hydrocarbon or an alicyclic hydrocarbon. Consequently, it is necessary either to incinerate the waste liquor discharged from the washing operation, after removal of the transition metal compound, or to distil it to recover the respective hydrocarbons. An improvement has therefore been desired for simplifying the preparation of the catalyst.
On the other hand, a solution polymerization is known in which the polymerization of ethylene is carried out with use of a Ziegler catalyst at such a high temperature condition that the polymer is formed in a form of a solution. For this solution polymerization method, it is desired that the polymerization is carried out at a temperature as high as possible in order to facilitate the removal of the reaction heat generated during the polymerization or to facilitate the separation of the polymer from the solvent and to maintain the melting temperature of the polymer thereby separated at a high level so that it can be pelletized with a minimum amount of energy, or to lower the viscosity of the polymer solution in the polymerization reactor and thereby to increase the concentration of the polymer solution. However, when ethylene is polymerized with use of the catalyst system proposed in the above mentioned Patent Publication at a polymerization temperature of at least 110.degree. C., especially at least 170.degree. C., at which the polymer forms a solution, there is a drawback that the catalytic activity is thereby considerably decreased. Consequently, the polymer obtained at such a high temperature has a drawback that it contains a substantial amount of the catalyst residue which tends to lead to the formation of rust on the molding machine or the colour degradation of the formed product. Under the circumstances, it has been desired to develop a catalyst system which does not undergo degradation of the catalytic activity even at a high temperature.